Im trying to create roads, that the units prefer walking on , by using Add path cost action of Pathfinding behavior.

First I made a negative Path Cost but the results are not what i expected. Then i tried to make a big square, to set the entire map PathCost 20 for example, and then set the road sprites to change the path cost to 0, but it seems the path cost can only be added, as the action name says. The only way it seems to work is to make sprites cover all the ground but the roads, but it will result in too many sprites for covering that area.